There is an increasing desire and necessity to convert maps and charts, such as are used in air and sea navigation, to digital signals. The motivation for digitizing the cartographic images is seen in the advantage in space in being able to store great numbers of charts electronically, in being able to electronically overlay radar or other indicia, and in being able to display portions or combinations of charts in decision making situations needing a great number of cartographic images available at once. Finally the updating of the many charts of the earth is more rapidly and economically done if in digital form.
The present craft of the cartographer has produced charts of great precision and clarity which, in combination with multicolor presentations in a two dimensional image, allows the viewer to rapidly process navigation or other graphic information from them. In the process of digitizing these images, with or without the use of mode filtering, many errors are encountered which greatly impair the usefullness of the image or destroy it completely. Among these are the loss of smoothness to borders between areas of different colors, such as lines. The border often is blurred by an off-color edge produced by digitizing errors. Half tone regions common on charts are often treated erratically due to the random phasing between half tone dots and scan pixels in the digitizing process. The definition of alphanumeric characters is often impaired in the scanning and digitizing process. Error colors also tend to creep into the digitized signal.
Some of these errors are apparent to the viewer when seeing the electronic display or printout of the digitized signals. They can be then manually corrected, but the effort is so time consuming as to make the use of digitized images very costly.